The invention relates generally to a magnetic gradient detector having two magnetic field sensors whose magnetic axes are parallel to an imaginary line, and located a certain distance from one another along the imaginary line. The sensor electrical outputs are connected with one another such that an electrical signal is created corresponding to the gradient of the magnetic fields present at the respective locations of the sensors.
Detectors of this general kind have been used in the past for the detection and measurement of magnetic field gradients, and as well for discovering hidden ferro-magnetic bodies such as mines, unexploded bombs, ships, etc., from the disturbances such bodies cause in the ambient magnetic field of the earth.
Because of their sensitivity and simple construction, harmonic detectors are frequently used. Their sensing element includes a magnetizable core which is excited to saturation by an applied magnetic alternating field of a specific frequency. When a magnetic field to be measured is superimposed on the exciter field, even harmonics of the specific frequency occur in a coil surrounding the core, the amplitudes of which are proportional to the magnetic field strength being measured.
In many cases, such as the locating purposes referred to above, magnetic field gradients of the magnitude of a few .gamma. (1 .gamma. = 10.sup.-.sup.5 oersteds) have to be resolved, while at the same time the gradient detector is subject to the full magnetic field of the earth, of about 0.3 oersteds. In order that, irrespective of any changes in its location, the gradient detector can suppress any influence from the undisturbed ambient magnetic field of the earth, it must meet two major requirements.
1. The two magnetic field sensors, which are electrically connected in opposition to one another, must possess an unchanging sensitivity over the whole range of magnetic field strengths to which they may be exposed.
2. The magnetically sensitive directions of the two magnetic field sensors, in other words their magnetic axes, must be parallel to within very small tolerances, and thus to the imaginary line referred to above.
While the first requirement can be met by compensating the average magnetic field of the earth at the location of both magnetic field sensors down to an almost zero level by the use of suitable reverse feedback coils, no fully satisfactory solution is yet known for the second problem. In a type of detector which has been used for many years, the two magnetic field sensors are installed in the upper and lower ends of a supporting tube, one end of each sensor being carried by a ball joint and the position of the other being adjustable by a setscrew. With this setup it is possible to obtain a very precise parallelism between the two sensors at a given moment, but the rigorous conditions of detection work cause them to slip out of adjustment, necessitating repetition of the paralleling operation from time to time. A great variety of factors can contribute to this misalignment, including temperature effects, impact or bending stresses, and many others. An indication of the level of parallelism required is given by pointing out that an angle as low as 1/100.degree. between the two magnetic field sensors can constitute a perceptible misalignment. Naturally, it is highly undesirable to have to constantly repeat the detector paralleling operation, since it interrupts the flow of work. Furthermore, paralleling has to be carried out in a location free from disturbance from the magnetic field of the earth, i.e., practically in the open country.